The deuteron — just like the proton — is smaller than previously thoughtThe deuteron — one of the simplest atomic nuclei, consisting of just one proton and one neutron — is considerably smaller than previously thought. This new research finding fits with a 2010 study in which, similarly, the proton was measured at the Paul Scherrer Institute and, likewise, a smaller value than expected was found. The result from 2010 formed the basis for what has been known since then as the proton radius puzzle.